1. Field of the Invention
The present invention relates to emulsion stability evaluation and the development of chemical demulsifiers.
2. Background of the Related Art
The inevitable creation and subsequent resolution of water-in-oil emulsions during the production and processing of crude oils are of significant importance in the oil field industry. These emulsions, which could typically be any combination of water-in-oil, oil-in-water, or complex emulsions, are very diverse in their nature and stability. The majority of oil field emulsions are resolved by the application of chemical demulsifiers in special processes under specific conditions. The stability of crude oil emulsions is influenced by a large number of variables and chemical demulsifiers are therefore specifically developed for each application, to achieve optimum economic efficiency.
Emulsion stability of water-in-oil emulsions encountered in the oil field industry can be evaluated using various methods, for example, the determination of droplet size and distribution by various methods, determining the amount of water resolved as a second phase, moisture analysis of the oil phase as well as more sophisticated methods such as interfacial rheology.
The use of Critical Electric Field (CEF) has recently been used to compare the stability of water-in-oil emulsions. With the CEF technique, a sample of water-in-oil emulsion is injected between two parallel electrode plates. A direct current voltage is applied between the two electrodes and increased in incremental steps, with continuous monitoring of the conductivity or electrical current through the oil sample. In response to the increasing applied electric field, the water droplets tend to align themselves to form agglomerated columns of droplets, which form a conducting bridge once a critical voltage (or electric field) has been reached. The strength of the electric field at which the sample shows a sharp increase in conductivity (increase in current through sample, between the two electrode plates) is recorded as the “Critical Electric Field”. By this method, relative emulsion stability is quantitatively compared in terms of the CEF-value and expressed in units of kV cm−1.
Demulsification processes may be described in terms of a few basic steps. Most often, these steps will include flocculation, coalescence and sedimentation. In some cases, aggregation or coagulation are preferred above flocculation to indicate specific qualities or properties of a pair or group of droplets. However, the term “flocculation,” as used herein, is considered to be the reversible formation of a droplet pair or cluster, with virtually no change in the total oil/water interface area and is intended to encompass, without limitation, aggregation, coagulation, and agglomeration. Coalescence is defined here as the complete association of two droplets to form a single droplet by rupturing the thin film that separated the two droplets. Sedimentation implies settling of the coalesced droplets or flocculated group of droplets under gravitational influence. Large droplet sizes, high differences in density and low viscosity of the external phase favours sedimentation of droplets, which can proceed along three main routes. First, individual droplets can settle without any flocculation or coalescence. Second, individual droplets can flocculate or coagulate to settle as flocculated pairs or clusters of droplets. Third, flocculated pairs or groups of droplets can coalesce to settle as a single larger drop or group of enlarged drops.
Stokes law dictates that the settling rate of a spherical droplet will be proportional to the density difference between the drop and the continuous phase as well as proportional to the square of the drop radius. This implies that the settling of a droplet aggregate will be faster than an individual droplet, but slower than its equivalent, single coalesced droplet.
In reality, any demulsification process will be a complex combination of steps and will be determined by a large number of variables. However, for practical purposes, emulsion stability is considered to depend on the degree of flocculation and coalescence. Since sedimentation can also proceed from the flocculated pair, both flocculation and coalescence can act as parallel pathways to sedimentation. In the case of a gravitational settling process, the rate of demulsification (or mechanism) is not practically controlled or limited by the coalescence process as the only route to emulsion resolution. The stability of water-in-oil emulsions, as observed under conditions of gravitational settling, therefore generally depends on the ability or tendency of the water drops to form bigger droplets by flocculation and subsequent coalescence
The majority of oil field emulsions are thermodynamically unstable. Thus, emulsion stability could in essence be described as a kinetic phenomenon, where emulsion stability is an expression used to describe the rate of phase separation for a given emulsion. Water-in-oil emulsions with high water settling rates are referred to as unstable emulsions, whereas emulsions with settling rates well outside the time domain of the observation, would be called stable emulsions.